1. Field of the Invention
This invention relates generally to bulk separation of gaseous mixtures and is particularly concerned with recovery of substantially pure hydrogen by selective adsorption from such mixtures of carbon dioxide and/or hydrocarbon gases in a pressure swing system. The system of the invention is also applicable to separation of CO.sub.2 from its admixture with methane.
2. Prior Art
Pressure swing cyclic systems designed for fractionation of gaseous mixtures of selective adsorption are well known in the art. In these systems one or more desired components of the gaseous mixture are separately recovered at a yield and purity depending upon the efficiency of the designed operation.
Illustrative of typical systems indicated to be especially useful in the recovery of hydrogen from gaseous mixtures with CH.sub.4 and/or CO.sub.2 are those described in U.S. Pat. Nos. 1,934,075; 3,104,162; 3,138,439; 3,142,547; 3,237,379; 3,788,037. Other patents describe in general, systems for separation of essentially binary gas mixtures or of multicomponent gas mixtures, asserted to be applicable in recovery of hydrogen from gas mixtures. Illustrative of these are the systems described in U.S. Pat. Nos. 3,102,013; 3,149,934; 3,176,444; 3,221,476; 3,323,288; 3,430,418; 3,619,984; 3,720,042. While certain of these disclosed processes are stated to obtain a hydrogen product of high purity, such high purity can be practically obtained only at a sacrifice in yield of recovered hydrogen product.
In U.S. Pat. No. 3,751,878 a cyclic adsorbent process is disclosed for the bulk separation of carbon dioxide from its admixture with methane and/or other gases including hydrogen, for the purpose of recovering the methane as such. The recovered methane product also contains the unseparated hydrogen and carbon monoxide.
Certain of these prior art patents typically employ a plurality of parallel columns arranged to undergo, in timed cycle sequence, the steps of (1) adsorption of the undesired component from the feed gas at elevated pressure and withdrawal of part of the effluent gas as desired product; (2) pressure reduction of the column to desorb the contained gas component(s); (3) purging of the column, generally with part of the primary effluent from (1); and repressuring the column with feed gas or with effluent from the first step. Among the different types of adsorbents suggested in the various patents, mention is made of molecular sieve zeolites of stated pore size, silica gel, activated charcoal or carbon, and others.
In the developments leading to the present invention, applicants have found certain of the key problems to be solved for efficient separation of gases by pressure swing adsorption to be:
(A) The manner of treating the void gas in the column after termination of the adsorption step. The handling of the void gas determines the ultimate extent of product recovery. At high operating pressure a large amount of the desired component of the feed gas remains in the void spaces of the column.
(b) The manner of regeneration or cleaning of the column following pressure reduction. The manner and extent of cleaning the column largely determine the quality of the recovered component.
Both (a) and (b) are critical for efficient and economic operation of the pressure swing adsorption schemes. Severe economic penalty results from loss of the desired component of the feed mixture in the void gas, particularly hydrogen, and from an inefficient regeneration step.
According to the process of the present invention, the several disadvantages of the process schemes of the prior art are overcome, and an operational system has been developed by which hydrogen can be recovered from a gas mixture at a purity of 99+% and at a recovery of over 95% and up to about 99%.